Provide an Empire clock with ETU resolution that is properly

Provide an Empire clock with ETU resolution that is properly
synchronized to updates (the one based on s_p_etu isn't synchronized):
(gamestr): New members game_turn, game_tick, game_rt.
(game_ca): New selectors for them.  Restrict tick and rt to deity for
now.  We'd need xdump update the Empire clock to make them safe for
players.
(game_record_update): New.
(update_main): Call it.
(game_tick_tick, game_tick_to_now, game_step_a_tick): New, not yet
used.